The invention relates to a plasma accelerator arrangement.
Plasma accelerator arrangements are known, in particular, in the form of what are known as Hall arrangements. Here, an ionizable gas, in particular a noble gas, is fed to a plasma chamber. The plasma chamber is essentially annular around a central longitudinal axis and is open in a main expulsion direction parallel to the longitudinal axis. Arranged at the foot of the plasma chamber, opposite to the expulsion direction, is an anode. A cathode, as electron source, is arranged outside the plasma chamber, offset radially with respect thereto. The walls of the plasma chamber consist of nonconductive material. A magnetic system produces in the plasma chamber a substantially radially oriented magnetic field by means of a first, inner and a second, outer magnetic pole. An electron stream emitted by the cathode is accelerated in the electric field between cathode and anode and fed to the plasma chamber and, in the plasma chamber, is deflected at right angles to the electric potential gradient and at right angles to the magnetic field lines by the radial magnetic field, and ionizes the propellant gas in the chamber. From the plasma formed in the process, the positively charged ions are accelerated in the expulsion direction. Some of the electrons emitted by the cathode are used to neutralize the ion beam.
An arrangement of this type, in which a special magnetic field design is intended to increase the efficiency and the lifetime, is known, for example, from EP 0 541 309 A1. Via oppositely polarized magnetic pole shoes, opposite and spaced apart radially from each other in the expulsion area of the annular plasma chamber, an inner and an outer coil system effect a substantially radial magnetic field in this area of the plasma chamber. The deeper area of the plasma chamber, with the annular electrode, is surrounded by a magnetic shield.
U.S. Pat. No. 5,847,493 A shows a plasma accelerator based on the Hall effect, in which an inner and an outer coil arrangement in each case produce two magnetic poles spaced apart in the expulsion direction, the respectively radially opposite poles of the inner and outer coil arrangement being oppositely polarized and in turn producing the substantially radial magnetic field. A similar plasma accelerator arrangement with partially conically shaped pole shoes is described in EP 0 982 976 A1.
DE 198 28 704 A1 describes a plasma accelerator arrangement in which, into a circularly cylindrical plasma chamber, a focused electron beam accelerated outside the chamber is introduced on the longitudinal axis and is guided on the axis by a permanent periodic magnet arrangement surrounding the plasma chamber cylindrically and having alternately aligned stages.
It is an object of the present invention to specify an advantageous plasma accelerator arrangement, in particular as a drive source for spacecraft.
The invention is described in patent claim 1. The dependent claims contain advantageous refinements and developments of the invention.
A significant aspect of the present invention is the fundamentally different configuration of the magnetic field which penetrates the plasma chamber, as compared with known arrangements. While, in the known arrangements, the magnetic system having a pole system on the inside and having a second pole system on the outside with respect to the toroidal plasma chamber, is substantially radially aligned and frequently closed via a magnetic return path behind the plasma chamber, the invention proposes a magnetic system which comprises, both radially on the inside and radially on the outside with respect to the plasma chamber, a magnet arrangement having a pole change in the longitudinal direction, that is to say in a direction parallel to the longitudinal axis of the arrangement. In particular, in a preferred arrangement, the pole changes of the outer and inner magnet arrangement can take place in the same direction, so that identical magnetic poles separated radially by the plasma chamber and belonging to the two separate magnet arrangements are opposite one another.
In a first embodiment, the magnetic system can be designed in one stage, in each case with a pole change for the outer and the inner magnetic system by means of opposite magnetic poles spaced apart in the longitudinal direction. At least one of the two magnetic poles in each case is located in the longitudinal direction in the area of the plasma chamber. Both poles of the single-stage magnetic system, spaced apart in the longitudinal direction, are preferably located within the longitudinal extent of the plasma chamber. Particularly advantageous is an arrangement in which the magnetic system is of multi-stage design with a plurality of successive subsystems in the longitudinal direction, each of which has an outer and an inner magnet arrangement and in which the successive subsystems in the longitudinal direction are alternately aligned in opposite directions. The radially opposite poles preferably have the same direction, which results in a particularly advantageous distribution of the electron drift.
Particularly beneficial is a plasma accelerator arrangement according to the invention in which there is still at least one electrode arrangement between the cathode and the anode in the area of the side walls of the plasma chamber, which arrangement is at an intermediate potential between cathode potential and anode potential. On such an electrode, electrons which have merely picked up energy from part of the potential difference between cathode and anode can be intercepted. The potential difference between cathode and anode can as a result be subdivided into two or more acceleration stages. Losses as a result of electron-wall collisions can be reduced substantially as a result. In particular, the electrical efficiency increases monotonically with the number of potential stages. The electrodes in the longitudinal direction are advantageously in each case placed between the pole ends of a magnetic system or magnetic subsystem. This results in a particularly beneficial course of electrical and magnetic fields.